A paper web and similar materials handled in sheet form are coated by applying onto a moving material web a coating mix which then is spread into an even layer onto the web surface with the help of a doctor blade. In the coating unit the material web to be coated passes between the doctor blade and a suitable backing member, conventionally a rotating roll. The blade doctors the excess coating mix from the web and levels the coating into an even layer on the web. In order to achieve a layer as even as possible, the gap formed between the web and the blade should have a constant spacing in the cross direction of the web over its entire width. The pressure applied to press the blade against the web should be high and constant over the entire length of the blade in order to attain an even spreading of the coating mix onto the web rotating at high web speeds.
For several reasons, the spacing of the gap between the material web and the doctor blade cannot be maintained exactly constant. During machining, the doctor blade and its frame are fixed to the machining unit base with strong clamps into a position simulating their operating position. Thus, the doctor blade frame is subjected to approximately the same forces as those exerted on it by its weight alone in its operating position. Despite the exact placement of the clamps, defects will develop during fabrication of the doctor blade and its frame causing a parallelism error to appear between the web surface and the doctor blade edge. As the doctor blade of the coating unit is pressed against the moving web, the blade is loaded with a linear force. Due to the pivotal support of the doctor blade frame by means of bearings mounted at both ends of the frame, the deflection induced by the linear load force at the center of the blade will be greater than at the supported ends, whereby the spacing of the blade from the web will be smaller at the edges of the web than at the center. Since the linear force exerted by the blade onto the web or the surface of the backing roll is smaller in the middle in comparison to the ends, any possible bumps on the web as well as variations in coating mix density and viscosity can lift the blade away from the web.
In order to alleviate the aforementioned disadvantages, several different solutions for the attachment of the doctor blade have been presented. In the prior-art constructions, a homogeneous loading of the blade over the entire web width has been attempted by means of a flexible blade and an adjustable blade holder element. In these embodiments, the blade is attached to the blade holder so that the blade can be pressed against the web by means of a flexible element, e.g., a pneumatically or hydraulically filled rubber hose, which extends across the entire length of the blade. Because of the equal pressure prevailing along the hose, the blade is pressed against the web with a constant linear force over the entire width of the web. The blade pressure against the web can then be adjusted by altering the pressure in the hose. These kinds of embodiments often use a doctor blade which is divided into smaller sections along its length. The advantage of this solution is a more flexible blade capable of higher compliance with the shape of the web and the roll.
Embodiments of the type described above are disclosed in, e.g., patent publications FI 842626, FI 57290, U.S. Pat. No. 3,748,686 and U.S. Pat. No. 4,367,120.
The described solutions carry several disadvantages. Because of the limited deformation capability of the flexible support member, this solution is incapable of compensating for large variations in the spacing between the blade and the web as well as in blade loading. The adjustment range of blade loading remains restricted and, if a higher coating speed is desired, the blade must be pressed against the web with an actuator element attached to the doctor blade. A higher blade loading results in an increased stiffness of the blade holder element, whereby the blade becomes incapable of complying with the web surface in the desired manner. The frame of the doctor blade must be constructed extremely stiff in order to make it possible to compress the flexible blade against the web. A blade consisting of a plurality of narrow sections in a comb-like manner is not compatible with all types of coating applications. If a smooth coat is desired, a continuous blade extending over the entire width of the web must be used, since a comb-like blade would allow excess coating mix to leak between the slits of the blade onto the web. The excess coating then forms streaks on the coat. Constructions based on flexible and adjustable doctor blade holders are complicated; blade replacement in the holders is cumbersome and the flexible members may break during blade replacement. The blade holder must be designed large and heavy in structure.